This invention relates to the field of vibration damping, specifically using extensional actuators for active vibration damping of relatively stiff structures such as boring bars.
In many metal cutting processes, the metal removal rate (MRR) is limited by the phenomenon of chatter, a regenerative vibration that is driven by workpiece surface undulations resulting from previous tool vibrations. Because resistance to chatter in the absence of workpiece flexibility is a function of the damping inherent to the tool structure, in many cases the use of feedback control to stabilize the primary modes of vibration should permit substantially higher metal removal rates.
Assuming a rigid workpiece, the precision that can be achieved in a metal cutting process such as a boring operation is determined by the static stiffness of the boring bar. Furthermore, the process stability is governed in large part by the bar's dynamic stiffness, which can be thought of as a combination of the static stiffness and the bar's inherent structural damping. Consequently, from both a precision and a stability viewpoint, it is desirable to use the stoutest tool available for a given operation. Many boring applications, however, necessitate the use of boring bars with high overhang ratios (L/D) for which it is difficult to achieve stability in the cutting process due to the inherent flexibility of the tool.
Since a tool's resistance to chatter vibrations is heavily dependent on the damping in the boring bar structure, numerous attempts to augment the bar damping by both active and passive means have been described. Most effective when placed in regions of high displacement near the bar tip, these devices commonly provide vibration suppression in only one direction, typically normal or tangential to the workpiece surface. Furthermore, complications associated with cutting zone interference and coolant application make these approaches cumbersome in practice.
Passive tuned mass dampers have been proven to be effective vibration dampers. See, e.g., Herzog, "Active Versus Passive Vibration Absorbers," Journal of Dynamic Systems, Measurement, and Control, Vol. 116, September 1994, PP 367-371; Cobb, "Design of Dampers for Boring Bars and Spindle Extensions," Master's Thesis, University of Florida, Gainesville, Fla, 1989. Passive tuned mass dampers suffer from two major drawbacks. First, the passive tuned damper is most effective when placed near the boring bar tip, exposing it to the caustic cooling fluids and possibly causing it to interfere with the cutting process. Secondly, to be effective, the passive tuned damper must be tuned to match the chatter frequency of the boring bar. However, the effective length of the boring bar is regularly altered during and between boring processes, and the passive tuned damper must be re-tuned to match the current frequency.
The limitations of passive tuned dampers can be circumvented by boring bars featuring nonlinear passive impact dampers. See, e.g., Fitzgerald, "A Comparative Study of the Lanchester Damper and the Segmented Slug Damper in Boring Applications," ASME Paper 81-Det-90, Sep. 20-23, 1981; "Kennametal Lathe Tooling," Catalog 4000, Kennametal Inc., Latrobe, Pa., 15650, p. 388. In these boring bars, a hollowed out section of the boring bar is filled with masses that dissipate vibrational energy through impact.
More recently, active tuned mass dampers have been mounted at the interior of the tool tip, avoiding the problems of interference with the cutting zone. Tewani et al., "Active Control of Machine Tool Chatter for a Boring Bar: Experimental Results," DE-Vol. 61, Vibration and Control of Mechanical Systems, ASME 1993, pp. 103-115. Electronic tuning of these dampers is more practical than the mechanical tuning of their passive counterparts, but the fundamental frequency of the bar must still be determined prior to initiating a cut. In addition, the internal geometry of the boring bar limits the size of the damper, making it inappropriate for many applications. Further, changing boundary conditions during a cut can shift the fundamental frequency and consequently reduce the effectiveness of the tuned damper.
Kennametal's recently introduced `Tuned Tooling` product line featuring internally mounted electronically tuned passive dampers overcomes many of the problems of conventional passive dampers. These devices augment cutting stability by suppressing the tool vibration at the first bending mode frequency, which is determined by a pre-cut impact test. But because frequencies tend to shift during cut, it is suspected that the effectiveness of this approach will be limited, though no field data is yet available.
In a project related to the Kennametal tuned tooling development, a boring bar featuring an active damper for increased functionality was developed under DARPA funding by a team led by Lucent Technologies. They developed an adaptive control circuit for their internal damper, producing impressive results in a floor demonstration. Test results indicated that the cutting conditions greatly impacted the tool performance, with some conditions favoring a damper orientation normal to the workpiece surface while others favored a tangential orientation. Thus, changing materials or cutting conditions can require removal and disassembly of the cutting head to properly orient the damper. This limitation is not problematic in a high production facility where a single set-up may be used to machine thousands of identical parts. But a more robust solution applicable to a wide range of cutting conditions was desired. Consequently, the Lucent team also produced an active tool holder capable of damping tool vibrations in two directions simultaneously. Good performance was demonstrated for a range of cutting conditions on different materials. The difficulty in designing a generic interface that enables the active clamp to be mounted to different machines limits the applicability of this approach.
Accordingly, there is a need for a vibration damping method and apparatus that can extend to damp vibration in more than one direction without requiring disassembly, that can readily accommodate varying tool dimensions, that does not interfere with tool tip operations and cooling, and that doesn't require custom interfacing.